This invention relates to a ceramic condenser and, particularly, to a reduction reoxidation type ceramic semiconductor condenser capable of having a large capacitance within a small sized body.
Conventional prior art ceramic condensers of the cylinder or plate type have a significant disadvantage in that their size is necessarily large if a large capacitance in the order of 1 .mu.F is desired.
For this reason, laminated ceramic condensers and semiconductor ceramic condensers were developed to provide a large capacitance with a small sized body. A small laminated ceramic condenser having a large capacitance can be produced; but, manufacture of such a structure requires that ceramic sheets be formed with inner electrodes laminated in several layers. The lamination is then subjected to a firing treatment and therefore the inner electrode material must be comprised of a costly noble metal of high melting point such as Au, Pt, Pd, and the like. The use of such materials is very expensive for a condenser of a large capacitance in the order of 1 .mu.F; thus, the foregoing type of ceramic condenser finds use in only very specific applications. On the other hand, the second type of condehser, i.e., the reduction reoxidation type semiconductor ceramic condenser is characterized in that a reoxidized layer is formed on a surface of the semiconductor ceramic and it serves as a dielectric body. This type of condenser has the advantage that it gives a large capacitance within a relatively small body. It also exhibits the nonpolar property of the other types of ceramic condensers, has superior heat resistivity and frequency response, has a small leakage current, and can be manufactured at a low cost. It does, however, have the problem that its area capacitance is at most in the order of 300 nF/cm.sup.2 and therefore must have a relatively large sized body to have a large capacitance in the order of 1 .mu.F.
One proposed solution to this problem relating to reduction reoxidation type semiconductor ceramic condenser is depicted in FIG. 1. In this structure a plate shape reduction reoxidation type semiconductor ceramic 1 has a dielectric layer 2 formed on the outer surface through a reoxidizing treatment. A non-ohmic electrode 3 is formed on dielectric layer 2, and an ohmic electrode 4 is formed on an exposed portion of semiconductor 1. In a condenser of the foregoing construction, a portion of semiconductor portion 1 is exposed and the ohmic electrode 4 is formed on the surface thereof, so that the area capacitance (Cs) will approximately double (in the order of 600-800 nF/cm.sup.2)that of a condenser whose whole surface is made of the dielectric layer produced through the reoxidation treatment. Conventionally, however, cutting and grinding processes are needed to expose the semiconductor portion 1 and this increases the number of process steps and, as a result, the production cost of the device is increased. Further, because the condenser body is plate shaped it is likely that cracking could occur in the cutting or grinding steps. In addition, where a number of semiconductor ceramics are processed simultaneously process control is difficult if the devices are not of the same thickness because the dielectric layer on the surface of a thinner body may not be completely removed.